a common class of medication prescribed to the elderly for the treatment of psychiatric disorders and behavioral and psychological symptoms of dementia (BPSD). However, in aged patients, the incidence and severity of the side effects such as experiencing extrapyramidal motor symptoms induced by APDs is increased. Although aged-induced changes in pharmacokinetics may contribute to the increased sensitivity to the side effects of APDs in the elderly, age-related pharmacodynamic changes at the target receptor level likely play a key role in the increased sensitivity to the side effects of APDs. However, the mechanisms underlying these age-related declines in receptor function are not well understood. Recently, we identified that histone modifications alter the therapeutic actions of a typical APD, haloperidol, in aged mice. In addition, our preliminary data in this application demonstrates that increases in the severity of extrapyramidal symptom-like side effects (motor side effects) in aged mice can be related to histone hypoacetylation of the dopamine 2 receptor (D2R) gene (Drd2) promoter that in turn decreases the expression of striatal D2Rs. Co- treatment with histone deacetylase inhibitors (HDACis) valproic acid (VPA) or entinostat (MS-275) restored the expression of striatal D2Rs and reduced age-related increases in the motor side effects of haloperidol. Our findings and preliminary results suggest that age-related histone modifications at the gene promoters of target receptors could affect APD action. In this proposal, we seek to confirm the novel epigenetic mechanisms underlying the regulation of APD action during aging and determine whether HDACis could be a candidate to improve APD treatment in the elderly. Our central hypothesis is that age-related increases in the motor side effects of APDs are due to histone hypoacetylation on their targeted receptor genes and that these epigenetic changes and their functional consequences can be reversed by co-treatment with HDACis. To test our hypotheses, first, we will verify that age-related histone modifications are one of the mechanisms underlying increased sensitivity to side effects induced by APDs. Then, we will identify the HDAC subtype(s) that contribute to histone modification and increase in the severity of APD-induced side effects in aged mice. Finally, we will evaluate the therapeutic benefits of HDACi and APD co-treatment that could reduce the severity of APD-induced side effects in aged mice and in Tg2576 mice, an animal model of Alzheimer's disease also being considered as a model of BPSD. The proposed study will advance our understanding of the mechanisms of age-related epigenetic alterations and their effects on APD action. This mechanistic concept will have implications not only for neuropsychiatric medication but also for other medications in geriatrics. Our study will serve as a guide to investigate epigenetic mechanisms on drug action with ultimate benefiting for the aged population.